Given a source and destination node, a routing protocol provides one or more network paths over which packets can be routed to the destination. The routing protocol computes such paths to meet criteria such as minimum delay, maximum data rate, minimum path length and so on. A routing metric that accurately captures quality of network links and thus aids in meeting such criteria is central to computation of good quality paths. The design of routing metrics for wireless multi-hop networks is challenging due to the following three unique characteristics of wireless links.
Time varying channels and resulting variable packet loss. The wireless links suffer from short term and long term fading and result in varying packet loss over different time scales. When the distance between the communicating nodes is large or if environment is obstacle rich and causes fading, the loss ratio of the link can be high. A routing metric should accurately capture this time varying packet loss.
Packet transmission rate. The packet transmission rate (or data rate) may vary depending upon the underlying physical layer technology. For example, 802.11a links have high data rate compared to 802.11b links. The data rate may also vary depending on the link loss characteristics when auto-rate control algorithms are used.
Interference. Wireless links operating in unlicensed spectrum suffer from two kinds of interference; namely uncontrolled interference and controlled interference. Uncontrolled interference results from non-cooperating entities external to the network that use the same frequency band but do not participate in the Media Access Control (MAC) protocol used by network nodes (e.g., microwave ovens and Bluetooth devices operating in 2.4 GHz ISM bands interfere with 802.11b/g networks in the same band). Controlled interference results from broadcast nature of wireless links where a transmission in one link in the network interferes with the transmissions in neighboring links. The interference of this kind depends on factors such as the topology of the network, traffic on neighboring links etc.